Synopsis A Sikorsky S-61N helicopter (C-FHFS), serial number61702, was climbing to pick up logs on the side of a hill, at about 85percent torque from each engine and approximately 2500feet per minute. It was pointing at the hill about 200feet away when the two pilots heard a loud bang and the rotor rpm (revolutions per minute) started to decay. The pilot flying turned the helicopter away from the hill, but the helicopter descended into the trees and came to rest, nose down and left side down, on a steep hillside. The helicopter was refuelled just before the accident flight. On impact, its fuel tanks ruptured, spilling a large quantity of fuel. There was no fire. The pilots suffered serious but non-life-threatening injuries. Ce rapport est galement disponible en franais. Other Factual Information The station of the pilot flying was encroached by impact damage. The area of the flight deck where the pilot not flying was stationed was not encroached, but the seat and restraint system of the pilot not flying broke free from its mountings. The pilot not flying was not wearing protective headgear. A post-accident teardown inspection of the engines revealed that both engines had damage signatures consistent with those found when engines are not operating at impact. There was no apparent malfunction that would cause the engines to shut down; however, the engines were equipped with overspeed governors that would shut the engines down in the event of an overspeed. These governors do not leave any indication that a shutdown was triggered by an engine overspeed. The main and tail-rotor blades demonstrated damage consistent with that found when there is little or no rotor rpm (revolutions per minute) at impact. The main-rotor transmission, which had accumulated approximately 710hours since its last overhaul, was also subjected to a post-accident teardown inspection, conducted at an approved Sikorsky main transmission overhaul facility in Richmond, British Columbia, under the supervision of TSB investigators. Initial disassembly of the front section revealed extensive wear and damage to both input freewheel units (IFWUs) (seeAppendicesA1 andA2). The IFWUs had been replaced, as a matter of practice, about 500 hours after the last main-transmission overhaul. The IFWUs that were removed demonstrated normal wear. The replacement IFWU (part number 61047-35000-060) had accumulated approximately 210hours of flight time and about 70engagements since installation. They were made up of new cams and rollers. During routine maintenance before the accident, there was no significant metal found in the main-rotor transmission oil filters. Also, no illuminated chip lights were reported. After the accident, during a teardown inspection of the main-rotor transmission, metal particles were found in the transmission, but chip sensors were not activated. A more detailed inspection of the metal found in the transmission oil filters revealed numerous flakes of bronze, some flattened. The bronze Oilite bushings (part number S6135-20459-101), on which the roller retainer sits, showed damage consistent with that found when they are comprised of material that cannot withstand the normal forces, or that have been exposed to an abnormally high level of vibrations. Other components of the IFWUs showed the following signs of instability, slippage, and skidding: The camshaft flats were dented by the rollers. The rollers had flat spots and were contaminated with bronze. The input gear housing was contaminated with bronze, and the inner surfaces were heavily damaged by slipping and skidding of the rollers. The roller retainers had dents along the lower surface on the back side of the tangs, scores and grooves on the inside between the roller pockets, and uneven wear in the pockets. The bronze Oilite bushings were extremely worn and showed cracks and bending. Further disassembly of the main transmission (see Figure1) revealed unusual gear wear patterns, fretting, and misalignment of the No.3 and No.4 bearing outer races relative to the steel liners. This was evident by the following: The spiral bevel pinion and the main spiral bevel gears were worn excessively; however, every fifth tooth had relatively little wear. Measurements taken during this disassembly to determine the appropriate shim thickness between the input housing and the transmission lower housing varied from 0.039inch to 0.048inch (a difference of 0.009inch). The required shim thickness was calculated to be 0.033inch at the last overhaul. The same measurements taken at the last overhaul varied from 0.024inch to 0.053inch (a difference of 0.029inch). Typical shim thickness in this application is 0.038inch to 0.044inch. At disassembly, the backlash between the spiral bevel pinion and the main spiral bevel gears was 0.022inch. At the last overhaul, the backlash was set at 0.016inch (the maximum allowable). Wear of the spiral bevel pinion and the main spiral bevel gears was not sufficient to account for the change in backlash measurements between overhaul and the post-accident teardown inspection. Dowels that align the cover and input housing were loose. Fretting, galling, corrosion, and scoring were found on the aluminum pilot liner around the No.4 bearing. The steel liner ring in the lower housing was found to be skewed. (The side opposite to the dowel pin, which retains the liner, had shifted from the centre of the transmission.) Fretting was found under the No.3 and No.4 bearing inner races on the input pinion (spiral bevel gear). The input housing casting was found to have contacted and fretted the lower housing casting near the No.6 bore. The input housing twist was measured to be right 0.110inch. (The maximum allowable off centre is 0.125inch, beyond which the housing must be scrapped.) The oversize liners machined for the bearing bores were of incorrect dimensions: the radius was 0.016inch instead of 0.040inch. The No.3 and No.4 liners were embossed with lettering and numbering stamped into the bearing outer races. Dimensions of the cams, rollers, gear housings, retainers, Oilite bushings, and gears were precision charted and found to have been manufactured according to Sikorsky design dimensions. Following the occurrence, selected main gearbox components from the accident helicopter were sent to Sikorsky for their analysis, and the TSB requested a copy of their report for purposes of the investigation. In early February, the TSB received a letter from Sikorsky, dated 05February2002, containing information based on their analyses. In October 2005, the TSB received a copy of an internal Sikorsky Materials Engineering Report dated 07February2001, prepared in response to the investigation of the S-61 accident, but not previously seen by the TSB. The content of this internal report is very similar to the content of the letter provided to the TSB. However, there are differences between the letter and the report, particular with regard to the Oilite bushings. In the letter, it is stated that One axial and one circumferential crack in the RH61350-20459-101 bushing had been mounted for metallographic examination; there is no mention of what was discovered. In the internal report, the above sentence is followed by The photomicrographic in Figure80 (axial crack section) shows several large pores in the oilite bronze. The pores shown in Figure81 (circumferential crack) appear to be more uniform and not as large as in the previous figure. In the letter, there is a finding(7) that reads as follows: The RH61350-20459-101 oilite bushing had many axial and circumferential cracks. However, in the internal report, there is a similar finding(6) that reads as follows: The RH61350-20459-101 oilite bushing had many axial and circumferential cracks. Microstructural and SEM exam revealed large pores and areas of apparent incomplete sintering. Other Sikorsky internal communication revealed that large pores and areas of incomplete sintering1 will allow the Oilite bushings to crush and crumble under loading associated with the application in the Sikorsky S-61 transmission. The incomplete sintering traces back to Oilite bushing (part number 703-06331-103) failures in 1999and2000. Analyses by Sikorsky of these failures indicated that Oilite bronze bushing does not meet radial crush strength requirements of MIL-B-5687D. This bushing does not have the same part number as the bushings in the accident helicopter, but the bushings are very similar and were made from the same materials. The original equipment manufacturer (Sikorsky) is responsible for the quality of the end product, in this case the Oilite bushing. There is no limitation on the size of pores permitted in Sikorsky's specifications for the material used to make the Oilite bushings nor is there any requirement that the material be completely sintered. The Sikorsky procedure to determine the shim thickness between the input housing and the transmission lower housing calls for a measurement between a fixture and the No.4 bearing inner race. The overhaul facility took 10measurements around the inner race and used an average to determine the shim thickness required. No upper or lower shim thickness limitations were specified in the Sikorsky or the overhaul facility transmission build-up procedures. There was also no requirement to carry out a gear pattern check.2 Information was not documented with regard to a vibration check that was conducted two days before the accident, as a result of an engine change. However, the technical logs indicated that the helicopter was maintained according to required standards. Power/topping checks were conducted about eight flying hours before the accident. One engine topped at 111percent torque and the other at 116percent, both well below the 123percent maximum the IFWUs are designed to hold. The pilots and maintenance crew reported no abnormal vibrations or noise before the accident. Operating procedures were in place to limit freewheeling and engagements of the IFWUs. Minimum torque values were adhered to during descents, cool-downs were accomplished at 98percent rotor rpm, and both engines were used to turn up the rotor system during starts. Weight and balance calculations are not performed for every lift during repetitive lift operations. However, log sheets of the loads carried indicate that they were normal for the type of operation and likely within the limitations set out in the helicopter flight manual.